Understanding and Application of Runner Systems

Time：2026-05-20 09:02:39 / Popularity： / Source：

Runner system is core of injection mold casting, responsible for smoothly, evenly, and with low loss, delivering molten plastic from injection nozzle to mold cavity. It directly determines product molding quality, production efficiency, and cost. A well-designed runner system achieves balanced filling, stable pressure, minimal waste, easy demolding, and defect-free operation.

I. Composition of Runner System

A standard runner system consists of four parts:
1. Main Runner: Connects nozzle and branch runners; its conical structure allows for rapid entry and smooth demolding.
2. Branch Runners: Melt distribution channels that evenly deliver melt to each cavity or multiple gate locations.
3. Gate: Final gate for melt entering mold cavity, controlling flow rate, holding pressure, and gate marks.
4. Cold Slug Well: Captures cold slug at front, preventing cold spots, short runs, and weld lines.

II. Classification of Runner Systems

1. Cold Runner System
- Principle: Runner solidifies as the mold cools, and ejects along with product after mold opening.
- Advantages: Simple structure, low cost, easy maintenance, suitable for small batches and multiple product varieties.
- Disadvantages: Generates runner waste, recycling increases costs; significant pressure and heat loss.
- Applications: Small to medium-sized molds, sample molds, small batch production parts.
2. Hot Runner System
- Principle: Heating keeps plastic molten within runner, resulting in no solidified material and no waste.
- Advantages: Zero waste, short cycle time, uniform pressure transmission, aesthetically pleasing gates.
- Disadvantages: High mold cost, requires temperature control and maintenance.
- Applications: High-volume production, high precision, transparent parts, thin-walled parts, multi-cavity molds.
3. Hybrid Runner System
Partial hot runner + overall cold runner, balancing cost and quality, used for complex structural parts.

III. Core Design Principles

1. Flow Balance: Multi-cavity molds should prioritize natural balancing (H-type, radial, symmetrical layouts), with equal-length and equal-section runners; unbalanced layouts should be manually balanced (adjusting diameter, length, and gates) to achieve synchronous filling.
2. Low Resistance and Low Loss: Flow paths should be as short as possible; corners should have a radius of ≥1.5mm; inner wall polishing should result in a Ra≤0.8μm; avoid abrupt changes in cross-section and sharp angles.
3. Reasonable Dimensions: Runner cross-sectional area ≥ product wall thickness; general plastic runners φ3–8mm, high-viscosity/glass fiber materials should use upper limit.
4. Smooth Venting: Venting grooves (0.02–0.04mm deep) should be provided at weld lines and end of filling to prevent air trapping, burning, and material shortage.
5. Easy Demolding: Main runner taper 1°–3°; cold slug wells should be used for ejection to prevent runner sticking to mold.

IV. Key Structural Design Considerations

1. Main Runner
- Conical, taper 1°–3°, length ≤ 60mm.
- Sprue sleeve ball diameter R = nozzle r + 0.5–1mm, leak-proof seal.
- Inlet diameter 0.5–1mm larger than nozzle.
2. Runner Cross-Section Selection
- Circular: Highest flow efficiency, lowest pressure loss, preferred choice.
- Trapezoidal/U-shaped: Easy to process, commonly used depth-to-width ratio 1:1.5.
- Rectangular: Fast heat dissipation, high resistance, used only in special scenarios.

Runner Cross-Section	Shape	Description
Circular		Slowest cooling rate, low heat loss and friction loss, but difficult to process.
Semi-circular		Good release properties, suitable for molds with complex parting surfaces, but high flow resistance, rarely used.
Rectangular		Advantages and disadvantages similar to semi-circular runners.
Trapezoidal		Higher volume than circular runners, easier to process and demold. Suitable for multi-plate molds, but produces more waste.

3. Gating Design (Common Types)
- Side gate: Universal, easy to process, suitable for most plastic parts.
- Point gate: Automatic cut-off, no marks, suitable for transparent/exterior parts.
- Submarine gate: Cut-off within the parting surface, preferred for automated production.
- Fan-shaped/Flake gate: Reduces flow rate, reduces flow marks and orientation stress.
- Needle valve hot runner: High precision, seamless, used for high-end appearance parts.

Side Gate
Illustration	Description
	Features	Side gate is the most common type of gate, often rectangular, hence also called a rectangular gate.
	Advantages	Easy to process, precise molding dimensions, easy post-processing of gate, and easy separation. Prevents backflow of plastic during injection.
	Disadvantages	Leaves residue on the surface, leaving marks on the product surface; unsuitable for appearance parts. High pressure loss.
	Applications	Suitable for most structural parts.

Fan-shaped gate
Illustration	Description
	Features	An extension of side-gate, widened at product.
	Advantages	Enables more even distribution of plastic throughout mold cavity, resulting in uniform filling. Reduces weld lines and other surface defects in finished product.
	Disadvantages	Large gate residue, difficult to remove in post-processing.
	Applications	Suitable for large, thin-walled parts.

Annular Gate
Illustration	Description
	Features	Annular gate, also called a disc gate, allows rubber compound in main runner to flow in a disc-like pattern in all directions.
	Advantages	It can stably fill product and reduce weld lines. Uniform filling, reduced deformation, and good appearance.
	Disadvantages	Large gate residue, difficult to remove in post-processing.
	Applications	Suitable for tubular products.

Point gate
Illustration	Description
	Features	A single dot on product surface allows direct injection of material.
	Advantages	Automatic material cut-off during mold opening, minimal residual marks.
	Disadvantages	Difficult processing, significant pressure loss.
	Applications	Suitable for exterior parts and products with numerous intersecting mesh patterns.

Submerged Glue
Illustration	Description
	Features	Submerged material enters product through a channel at an angle from runner.
	Advantages	Automatic material cut-off upon mold opening, minimal residual marks, easily automates semi-automation.
	Disadvantages	Difficult to process, significant pressure loss. Not suitable for soft or poorly tough materials.
	Applications	Suitable for high-quality products or products with deep ribs.

Horn-shaped injection mold
Illustration	Description
	Features	It bends into a channel from runner to product.
	Advantages	Automatic material cut-off upon mold opening, minimal residual marks, easily automates semi-automation.
	Disadvantages	Difficult to process, significant pressure loss. Difficult ejection; unsuitable for soft or poorly tough materials.
	Applications	Suitable for high-profile products or products with deep ribs.

Hot runner
Diagram		Description
	Features	A channel bends into runner to reach product.
	Advantages	a. Reduced waste, b. Shorter cycle time, c. Reduced pressure loss, d. Controlled gate marks, e. Controlled plastic flow
	Disadvantages	a. High cost, b. Complex structure, c. High maintenance costs
	Applications	Most products

4. Cold slug well
Located at the end of main runner and end of branch runners, with a volume 1–1.5 times cross-sectional area of runner, effectively intercepting cold slug.

V. Material and Process Compatibility

- Low viscosity (PP/PE): Smaller runner, smaller taper (1°–1.5°).
- Medium viscosity (ABS/PS): Standard runner φ4–6mm.
- High viscosity (PC/PMMA): Larger runner, larger taper (2°–3°), to avoid shear overheating.
- Glass fiber reinforced: Thicker and polished runner to reduce wear and stagnation.

VI. Runner system and mold coordination

- Matching with cooling system: Water distribution near runner ensures uniform cooling and prevents warping.
- In conjunction with ejection system: Cold slug well features a Z-shaped pull mechanism to ensure smooth ejection from runner.
- In conjunction with venting system: Venting is required opposite gate and at weld joint.

VII. Design Summary

Runner system is "vascular network" of mold. Core design principles are: balanced filling, low resistance and smooth flow, precise dimensions, material compatibility, easy demolding, and cost reduction. Cold runners are chosen for small batches, while hot runners are chosen for large batches and high precision. Prioritize circular cross-sections, symmetrical layouts, and rounded transitions. Combined with CAE mold flow analysis verification, this maximizes molding stability and yield.

Gud Mould Industry Co., Ltd